1. Field to Which the Invention Pertains
It is old in the art to deep draw paperboard into containers for food. However, there has hitherto been restrictions on the type of containers that could be produced by known processes. These restrictions have existed since the prior art processes have not enabled the production of certain types of containers without substantial tearing of the paperboard substrate, particularly at the rounded edges of the deep drawn containers. Specifically, it has hitherto been thought that containers with a depth greater than about 1.375" a radius of curvature of its bottom corners of less than about 7/8" and sidewalls with an angle of inclination of less than about 20 degrees from vertical could not be obtained by a deep drawing operation without substantial tearing of the paperboard substrate.
The tearing problem associated with deep drawing a paperboard blank to a depth greater than about 1.375" is particularly acute when the radius of curvature is less than about 5/8" and the sidewalls are less than about 17 degrees from the vertical.
The tearing associated with drawing operations was referred to in U.S. Pat. No. 2,270,185 to Dulmage. Specifically, the Dulmage patent teaches drawing thermoplastic blanks which may contain paper spacer sheets. The patent teaches that the paper spacer sheets sometimes tend to rupture under the stress of the drawing operation.
The above noted restrictions on prior art deep drawn containers have been particularly disadvantageous since such containers are most desired for use as containers for food. One reason why these containers are so desired is that they have a relatively large volume for the amount of storage space required for the container. Additionally, since the corners of such containers are essentially square the containers may be stored with other such containers, i.e., on grocery shelves, with a minimum amount of wasted void space between adjoining containers.
Additionally, deep drawn containers have advantages over other prior art containers which require separate folding steps and external fastening or bonding means such as described in U.S. Pat. No. 3,511,140 to Hoyrup. Specifically, deep drawing processes do not require multiple steps, and the containers produced do not contain potential sites of weakness at the fastening or bonding sites. It, therefore, would be particularly advantageous to provide deep drawn containers which have a relatively large volume while requiring a minimum amount of storage space.
The invention disclosed herein relates to deep drawn paperboard containers having a depth greater than about 1.375"; a bottom with a radius of curvature less than about 7/8" and, preferably, less than about 5/8" and rounded sidewalls with an angle of inclination less than about 20 degrees from vertical and, preferably, less than about 17 degrees from the vertical.